SOLVED. Adult Sci Fi. Person hired to take a census of the population in a US state, looks for trends. Notices a small but ongoing population growth from a small city. His boss see it as a statistical error, but he travels to the region/small city. [s]

[Sascha]

I guess that the book was published in the 1980’s, 1990’s. It’s not about space ships or technology, rather a slow but ongoing invasion of aliens from outer space, who look like completely normal people.

[Genesistrine]

Short story, but Hobson's Choice by Alfred Bester?

“Oh, hell,” Grande answered. “You’re not supposed to be finding anything. We’re in between statistics until the war’s over.”

“I was leafing through the Interior Department’s reports. D’you know our population’s up?”

“Not after the atom bomb it isn’t,” said Grande. “We’ve lost double what our birthrate can replace.” He pointed out the window to the twenty-five-foot stub of the Washington Monument. “There’s your documentation.”

“But our population’s up 3.0915 percent.” Addyer displayed his figures. “What about that, Mr. Grande?”

“Must be a mistake somewhere,” Grande muttered after a moment’s inspection. “You’d better check.”

“Yes, sir,” said Addyer scurrying out of the office. “I knew you’d be interested, sir. You’re the ideal statistician, sir.” He was gone.

[Sascha]

By golly, that's the one! Ah, so many thanks from me, this is awesome! A classic science fiction short story. Now a nice cup of tea, and some nostalgic reading.

You're the best!